Ethiopian Prime Minister Meles Zenawi , a strongman in the troubled Horn of Africa and a key United States ally , has died , his government announced Tuesday .

Meles had not been seen in public in months , sparking intense international speculation about his health .

The government revealed neither the cause nor the location of his death Monday at the age of 57 .

Meles came to the forefront as a leader of a guerrilla insurgency against dictator Haile Mengistu Mariam in 1991 and cemented power in the ensuing decades .

Seen by admirers as a force for stability in a region with Islamist insurgencies in Somalia and Yemen and a history of famine , he was also criticized for cracking down on political opposition and the press .

`` He came to power at the barrel of a gun , but he made the transition from rebel leader to political leader very quickly , '' said Ayo Johnson , a writer on Africa and director of Viewpoint Africa .

But he never overcame his `` mindset as a rebel leader , '' and his democratic credentials were `` poor , '' Johnson said .

`` The West turned a blind eye to many aspects of his game that were not up to scratch , '' he added .

That 's partly because of his action against Islamist movements in the region , including an invasion of Somalia in 1998 , Johnson said .

`` He supported everything that the United States wanted to do against terror , '' he explained , such as give American drones based in the region permission to use Ethiopian airspace on their way to targets in Somalia .

And `` he was able to address the fundamentals -- health care , education , reducing the poverty rate , '' Johnson said .

U.S. President Barack Obama praised Meles ' `` unyielding commitment to Ethiopia 's poor '' in a statement Tuesday , citing his `` personal admiration for -LRB- Meles ' -RRB- desire to lift millions of Ethiopians out of poverty through his drive for food security . ''

But in June , U.S. Sen. Patrick Leahy , a Vermont Democrat , highlighted a different aspect of Meles ' rule -- the case of journalist Eskinder Nega , who was then before a court in Addis Ababa .

While praising the prime minister 's work in `` containing the real threat of terrorism in the region and making gains against the region 's recurring famines , '' Leahy accused him of `` trying to silence those who do not toe the official line . ''

His Ethiopian People 's Revolutionary Democratic Front took all but one seat in parliament in elections in 2010 , Africa expert Jason Mosley pointed out in the wake of Meles ' death .

`` Nearly 200 people were reportedly killed , and tens of thousands detained '' in the previous elections in 2005 , said Mosley , who is with the Chatham House think tank in London .

Last year , Ethiopia found two Swedish journalists guilty of supporting terrorism and sentenced them to 11 years in prison .

Meles died at 11:40 p.m. Monday from an unspecified infection , spokesman Bereket Simon said . Deputy Prime Minister Hailemariam Desalegn is now in charge .

There will be no elections before the next scheduled ballot in 2015 , Bereket said .

Meles had been scheduled to step down in 2013 as part of a transition process , Mosley said , but questioned whether he genuinely intended to relinquish power .

Meles was out of the country when he died and members of his family were with him , according to Bereket . He did not say what country Meles ' body was in or when it would return to Ethiopia , except that it would be soon .

The spokesman acknowledged the prime minister had been sick for some time but did n't immediately seek treatment .

The news came almost a week after the government said Meles was `` recovering well '' after treatment for an unspecified illness .

Bloggers launched a counter of the number of days he 's been missing , while citizens took to social media to discuss his whereabouts and exchange conspiracy theories .

The secretive nation had released little information about his whereabouts , prompting rumors and opposition claims that he was dead or facing a life-threatening illness .

The government held a news conference last month and announced Meles received treatment for an unspecified illness .

His absence was more evident last month when Ethiopia hosted an African Union summit in its capital of Addis Ababa . Meles , a key player in talks on the tensions between Sudan and its rival neighbor South Sudan , did not attend .

Former U.N. Secretary-General Kofi Annan said Ethiopia under Meles had `` played a key role in both the region and the African continent . ''

`` I hope that his successor will continue to be a driving force on a wide range of issues , from brokering peace negotiations to shaping development relationships , '' Annan said .

Ethiopia , a key Western ally often lauded for effective use of aid money , is surrounded by unstable nations such as Somalia and Sudan . Meles has been credited with working toward peace and security in the region , and the Ethiopian army sent peacekeepers to battle the Islamic extremist group Al-Shabaab in Somalia .

People we 've lost in 2012 : The lives they lived

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Obama praises Meles Zenawi 's `` unyielding commitment to Ethiopia 's poor ''

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He supported `` everything the United States wanted to do against terror , '' an expert says

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Critics point out his efforts to stifle political opposition and silence journalists

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He came to power as part of a guerrilla insurgency against a dictator